The arc tube for a high pressure sodium (HPS) lamp typically comprises a translucent polycrystalline alumina (PCA) or yttria tubular envelope sealed at either end with a ceramic button fabricated from a similar material. Passing through this button is a feedthrough member (also referred to herein as "inlead") comprising niobium. The feedthrough member may be in the form of a tube or a wire. The feedthrough member may further comprise other metal additives such as, for example, zirconium. The feedthrough member performs the function of electrical inlead and electrode support. The end structure of the arc tube is hermetically sealed by means of a frit material. Frit materials typically comprise oxides of calcium and alumina. Frit materials may further include oxides of barium, magnesium, boron, strontium, beryllium, and/or yttrium.
Arc tubes typically are constructed in one of three conventional designs: the monolithic design, the hat design, or the disk design. Examples of these three conventional designs are described and illustrated in U.S. Pat. No. 4,713,580 issued to Schoene, which patent is hereby incorporated herein, to the extent necessary, by reference.
The frit material used to form seals in the arc tube of a high pressure metal vapor discharge lamp must have a composition which does not react with the components of the fill gas. Additionally, the thermal expansion properties of the frit material should be within certain tolerances of those of the materials used to form the envelope , seal button, and feedthrough member so that the seal formed will not crack upon thermal cycling. For practical reasons during sealing, it is desirable to minimize the melting point of the frit sealing material.
High pressure metal vapor discharge lamps of the high pressure sodium type operate at seal temperatures of about 700.degree. C. Although such lamp has very high luminous efficiency, the color of the light output is not satisfactory for many applications. Thus, there is a need to improve the color of such HPS lamps.
Because of the low color rendering index (CRI) and color temperature of HPS lamps, much research effort has been directed to improving the color of the lamp light output. One technique for improving color has been to increase the sodium pressure of the lamp which has the effect of increasing the overall CRI Examples of this technology for lamps with a CRI of about 60 are described by Bhalla (J. Illuminating Engineering Society, Vol. 8, pp 202-206 (1979)). These lamps only increase the correlated color temperature of sodium lamps from about 2100.degree. K. to about 2250.degree. K. This small improvement in color temperature has not been of sufficient magnitude to compensate for other disadvantages associated with this technique. Thus, the resulting lamp has not been well received in the market.
Another approach has been to raise the sodium pressure still further, which raises color temperature to about 2700.degree. K., but the drop in efficacy for such a lamp is precipitous. To increase sodium pressure, the seal temperature must be increased. Sealing frit materials developed for this purpose are described in U.S. Pat. No. 4,501,799. These frit materials have melting temperatures in excess of 1600.degree. C. Such temperatures are much greater than those of conventional sealing frit materials which have melting temperatures of about 1250.degree. C. Further, the rare earth elements included in these frit materials cause these frit materials to be more costly than standard frit materials which include alkaline earth oxide components.
U.S. Pat. No. 4,409,517 issued to Van Der Sande et al. describes achieving improved color in discharge lamps employing ceramic arc tubes which include metal halide fills. To avoid the reaction of the halide components of the fill with the niobium feedthrough, Van Der Sande et al. teach applying a halide resistant coating to that portion of one of the upper inleads which is in contact with the lamp fill. The coating protects the inlead from reaction with the halide vapors.
Another technique for improving the color of high pressure sodium discharge lamps is to include additional radiating elements in the fill. This technique was originally described in U.S. Pat. No. 3,521,108 issued to Hanneman. These lamps typically operate with seal temperatures about 1000.degree. C. Such lamps often experience premature failure.
One of the reasons for failure of high pressure metal vapor discharge lamps of the high pressure sodium type is believed to be caused by reaction between the frit material and the sodium component of the lamp fill. Complete elimination of sealing frit material from the arc tube assembly of such type of lamp is disclosed as a solution to this problem. See U.S. Pat. No. 4,545,799, of Rhodes et al. This patent describes assembling an arc tube envelope made from unsintered, compressed ceramic powder, an insert made from unsintered, compressed ceramic powder, and inlead, and sintering the assembly in a single step without interposition of any frit whatsoever. Such a process requires both the electrodes be inserted through the feedthrough and welded thereto and the fill be added through the hollow inlead after the sintering operation. Neither of these fabrication constraints is desirable.